Small and medium-sized enterprises (SMEs) account for 99 percent of all the businesses globally and generate 50-60% of employment. Although sustainable environmental performance is a primary imperative, these SMEs remain under-represented in sustainability writings when compared to large corporations, despite the fact that they generate large amounts of industrial waste and emissions. This systematic review aggregates empirical data on determinants and outcomes of environmental performance in SMEs. In accordance with PRISMA guidelines, we conducted a search in Scopus and Web of Science databases for peer-reviewed articles (2015-2025) that returned 63 studies out of 1,077 initial records. Six determinant categories were identified through thematic analysis: organizational (environmental leadership, green human resource management), stakeholder pressures (regulatory, market, institutional), innovation and technology (green innovation, digital systems), resource-based (human and financial capital), strategic orientation, and supply chain collaboration. Reduction of emissions, environmental outcomes, certification, and resource efficiency were the most frequently occurring, having varying operational and financial impacts. Methodological quality was 7.8/10 on average (82.5% ≥8). Studies applied predominantly quantitative methods (78%) in developed (44%) and developing (41%) economies. The most important gaps identified are longitudinal determinant interactions, SME heterogeneity, and resource constraint prevention strategies in diverse contexts.